If you have a large library of FLAC encoded music and want to put it on a device with limited storage like an Android tablet or phone, Opus is a great choice. Just make sure you have a player like VLC that can have support for the codec.

To convert FLAC files to Opus you will need opus-tools installed. Then you can use the opusenc program.

for i in *.flac; do opusenc --bitrate 256 "$i" "$i.opus"; done

You can experiment with the bit rate if you like, 256 produces a quality that is probably indistinguishable from the FLAC file for most people.